Granular aluminum and method of making the same



Patented June 20, 1939 UNITED STATES PATENT OFFICE GRANULAR ALUMINUM AND METHOD OF MAKING THE SAME No Drawing. Application April 10, 1936, Serial No. 73,640

25 Claims.

Our invention relates to the production of an improved granular aluminum that may be advantageously used in metallurgical reactions,

more particularly as a reducing agent.

Metallurgical reactions performed by means of the well-known aluminothermic process require metallic aluminum in a finely divided form for mixing with the oxide of the other metal to be reduced. Aluminum powder such as is used for paint pigment is not generally used for metallurgical reactions on account of its high cost, and aluminum shot formed by pouring molten aluminum into water is not satisfactory on account of its coarse size and compactness of form, thereby giving slow and incomplete reactions.

The preferable form of aluminum for these reactions is known as grained aluminum, and is produced by breaking up the metal mechanically just as it is passing from the liquid to the solid state. Particles formed in this way are rough and cracked, if the breakingup has been accomplished at just the right temperature, and if they are also not too coarse, they react vigorously with metallic oxides.

We have now discovered that a new and improved form of grained aluminum may be readily produced by adding certain salts and compounds to the solidifying aluminum while it is being stirred vigorously; I v

We have also discovered that the vigor of the aluminothermic reaction in which this product is used may be controlled in a convenient way by adjusting the amount and kind of the compounds which are mixed with the aluminum while graining it.

Although these discoveries have been made by us after experimentation and tests, and are now utilized by us in the manufacture of ferrotitanium by the aluminothermic process, they are of course applicable to grained aluminum in general, and for whatever metallurgical reactions that product may be applied to, such as the manufacture of nickel-titanium, ferro-chromium, ferrozirconium, ferrocolumbium, ferromolybdenum, etc., or in the direct manufacture of alloy steel by the reduction of the oxide of the alloying element such as chromium, zirconium, titanium, columbium, etc., by means of our improved grained aluminum in the steel-melting furnace.

As one example of the application of our improved methods for makingand using the improved form of grained aluminum. which is the subject of our invention, the. particular kind intended for the manufacture of ferrotitanium will now be described.

Example A.Alumlnum of ordinary commercial purity was first melted at 1400 F. or higher in an oil-fired tilting crucible furnace, and at 5 convenient intervals 40 pounds were poured out and transferred to an iron pot over which a suitable stirring device was mounted. Our preferred type of stirring device consisted of a vertical shaft with a curved plate on the bottom end, the 10 curve being fitted approximately to the contour of the pot, and having a deeply toothed edge. The shaft was capable of being rotated, and also capable of being raised or lowered relatively to the pot. 15

After the.molten aluminum was poured into the pot, the stirrer was placed in operation to agitate the molten aluminum while cooling, and a mixture consisting of '7 pounds of powdered synthetic cryolite (a mixture of sodium fluoride and aluminum fluoride) and 7 pounds of powdered rutile (containing about 81% T102, 3% SiOz and balance FezOs) was added to the pot.

The cryolite we used contained approximately:

When these powders were thoroughly mixed with the partly frozen aluminum and the entire batch had cooled to a very dull red heat, around 1200 F., it was charged a little at a time into a suitable swing-hammer mill which effectively completed the granulation.

After passing through this mill the aluminum was solid, and the mixture was thrown on an inclined screen of inch mesh, the oversize being returned to the pot for softening and breaking up with the next batch.

In this way all the aluminum was eventually utilized, the only losses being by oxidation in melting, and in dust from the hammer mill. These usually averaged about 9%.

Various salts or metallic compounds (milled to about mesh or finer), such as sodium chloride, potassium chloride, sodium carbonate, magnesium chloride, ammonium chloride, and mixtures of them, have been tried by us in place of cryolite in our improved methods, but cryolite has been found most satisfactory for the graining process. The graining process was also tried without any salt addition, but it was extremely difficult and laborious to secure fine grains in that way, more than 50% oversize invariably being obtained.

As examples of the use of various salts to assist in the graining of aluminum by improved methods the following experiments may be mentioned. Fifteen pounds of aluminum were melted in the graining pot, and the dross was skimmed off. The molten metal was stirred 9 minutes mechanically, while cooling, and during that time 4 pounds 7 ounces of powdered common salt (commercial sodium chloride) were added gradually. This salt had been powdered by milling for one and one-half hours in an iron ball mill. After this addition, stirring by hand was continued until the lumps were too cold to be broken up. The product was then washed with hot water, and when thoroughly washed and dried, it was screened through an 8 mesh screen. The aluminum remaining on the screen was 6 pounds 6 ounces while 8 pounds 7 ounces passed through. The production of fines was thus 56%.

When this method was repeated using 3.75 pounds of powdered cryolite (a mixture of 43% aluminum fluoride, 56% sodium fluoride, and 1% sodium silico-fiuoride, moisture, etc.), instead of the sodium chloride, and without washing of the product, we obtained 16 pounds 6 ounces fines through an 8 mesh screen, and only 1 pound 12 ounces of oversize, or a yield of 87.5% fines.

When repeated by using for the graining agent 5.25 pounds of a mixture consisting of 40% soda ash (commercial sodium carbonate), 22% commercial sodium chloride, and 38% commercial potassium chloride, the method yielded a product which, after thorough washing and drying, consisted of 10 pounds 14 ounces of granular aluminum finer than 8 mesh and 4 pounds 10 ounces of coarser particles, thereby indicating a production of 72.5% fines.

Other mixtures of salts were also tried with much inferior results, and we therefore decided that cryolite was more satisfactory than other salts having lower melting points.

The use of cryolite to assist the graining has the further advantage that its presence with the grained aluminum improves the reactivity of the latter when it is used as a reducing agent in metallurgical work. Cryolite, in fact, has been found to be practically indispensable as a flux when aluminum is so used, and the advantage of having this flux intimately associated with the aluminum grains is therefore obvious. By our improved methods the aluminum grains become coated with the cryolite flux instead of carrying a coating of refractory aluminum oxide which is unavoidably formed when the graining operation is attempted in air without the aid of any salt such as cryolite.

The amount of cryolite used has been varied between zero and 27.4% of the total mixture. When less than about 10% was used, the graining operation was difficult (unless other compounds such as rutile were used in addition) and when more than about 20% cryolite was added, the aluminothermic reactions in which the grained aluminum was used were too violent.

'For example, in making ferrotitanium with this improved grained aluminum, a reaction, in which 138 pounds of grained aluminum containing 24.5% of cryolite was used, lasted only 1% minutes, and yielded 15 pounds of metal; while another one made with the same amount of the same kind of mix, except that the grained aluminum contained only 13.5% of cryolite lasted nearly 2 minutes and yielded 66 pounds of metal.

Since the manufacture of grained aluminum was facilitated by the addition of over 20% of powdered nonmetallic compounds (such as cryolite), while stirring the freezing metal, and the presence of so much cryolite in the grained aluminum made it too reactive for more effective use, the substitution of a less active ingredient for some of the cryolite was tried.

The most useful ingredient for this purpose would naturallybe the oxide of the metal for the reduction of which the grained aluminum is to be used. Rutile, for instance, has been used when ferrotitanium was to be produced, and pure titanium oxide when nickel titanium was the final objective. Similarly other oxides may be used when other alloys are to be made. The amount of rutile or other metallic oxides which we have used has ranged ordinarily from zero to about 17%, but for special purposes, such as the direct manufacture of alloy steel by the addition of grained aluminum and the oxide in a steel-melting furnace, any proportions up to two or more of oxide to one of aluminum may be advantageously employed. The proportions noted in Example A, namely 13% each of cryolite and rutile, We have found to be very satisfactory for grained aluminum to be used in the manufacture of ferrotitanium by the aluminothermic process; such graining operation is easy, and an average reaction using 130 pounds of the product and lasting about 2 minutes gives a yield of to pounds of high quality alloy.

To make an aluminum-rutile mixture with approximately equal parts of each in a fine-grained form, 30 pound batches of molten aluminum were poured off into the graining pot, and to each one 5 pounds of cryolite and 25 pounds of fine rutile were added while stirring. The analyses of these materials have been heretofore given in this example.

In order to obtain proper graining of the aluminum and also to avoid the formation of course lumps from too rapid chilling, it was found necessary to preheat these large additions of rutile. This was most conveniently accomplished by placing the rutile and cryolite in the graining pot first, and heating them there to about 800 or 900 F. The molten aluminum was then poured on top, and after stirring vigorously while cooling, and also passing through the swing-hammer mill when cooled to a pasty condition, a wellgrained mixture was obtained. From two batches of the quantities mentioned, the yield was 101 pounds of product passing an 8 mesh screen, with 7 pounds oversize.

Two other batches were made using even more rutile, or in the proportion of 40 pounds of preheated fine rutile, 3 pounds of cryolite, and 20 pounds of molten aluminum, This also gave a good granular mixture, 97 pounds from the two batches passing through the 8 mesh screen, with 11 pounds oversize, the balance being chiefly dust loss because the rutile was rather fine.

It is thus evident that our improved graining methods are practical for the production of a mixture containing twice as much, or more, of a reducible oxide as of the granular aluminum reducing agent, when such a mixture is desired for furnace additions.

Example B.--The following example illustrates the use of our new granular aluminum in the manufacture of nickel-titanium. Aluminum was melted, transferred in 39 pound batches to the graining pot, stirred and passed through the swing-hammer mill as described in Example A, but the addition consisted only of 9 to 10 pounds of cryolite for each batch. This operation was repeated for batches which yielded 86% of the original materials as an aluminum-cryolite mixture through an 8 mesh screen, the missing 14% being accounted for by dross formed in melting, dust losses, and oversize.

Then 41 pounds of this product mixed with black nickel oxide (containing over 99% NiaOa, and less than 1% F6203, CuO, and other impurities), and of rutile containing 94.7% T102, 2.16%

F6203, 0.97% SiO2, and 2.17% CaO, A1203, and other impurities, was milled to a fine powder in a ball mill, then packed in a. suitable container, and ignited.

After the reaction 35 pounds of alloy containing 28.8% titanium, 7.64% aluminum, 2.8% iron, 1.11% silicon and balance nickel (about 59.65%) were recovered, whereas by using commercial finely divided aluminum plus a proportional amount of cryolite in a similar mix and process, only 32.5 pounds of alloy containing 26.7% titanium were obtained.

Example C.-The following example shows the use of our improved methods in the manufacture of ferro-chromium. Grained aluminum was prepared exactly as described in Example A, except that in place of 7 pounds offine rutile .per batch of 40 pounds of aluminum, 7 pounds of finegrained chrome ore were used. The cryolite was used in the same way as with rutile. The chrome ore contained approximately 47% CrzOs, 21% FeO, 15% MgO, 11% A1203, 5% Si02, and 1% CaO. This gave just as good results as rutile in graining, and from pounds of molten aluminum, 14 pounds of cryolite and 14 pounds of chrome ore, pounds of the grained mixture passing through an 8 mesh screen were recovered with 9.5 pounds oversize.

The fine product was used as a reducing agent for making ferro-chromium in the following way: A mixture consisting of 65 pounds of the lowgrade chrome ore as before described, 25 pounds of the grained-aluminum with cryolite and chrome ore, 3 pounds of calcium silicide (containing about 30% calcium, 3% iron, and balance silicon), and 5 pounds of cryolite was milled in an iron ball mill for 15 hours, and then 3 pounds of magnesium shavings and 10 pounds of sodiuni chlorate were added, and then mixed for half an hour longer in the same mill. This mix was packed in a suitable container and ignited. A violent reaction occurred, and the alloy recovered weighed 21.5 pounds and contained 50.32% chromium, 35.73% iron, 8.54% silicon, and the balance chiefly aluminum.

Example D.The following example shows the use of our improved methods in the manufacture of ferro-aluminum-zirconium: Grained aluminum was prepared exactly as described in Example A, except that in place of 7 pounds of fine rutile per batch of 40 pounds of aluminum, 7 pounds of fine zirconium oxide was used. The cryolite was used in the same way as with rutile. The zirconiacontained approximately 95% ZIOz, 4% SiOz, and 1% impurities, including TiOz, F8203, A1203, etc. This gave nearly as good results as rutile in graining, and from 80 pounds of molten aluminum, 14 pounds of cryolite and 14 pounds of zirconia', 91 pounds of the grained mixture passing through an 8 mesh screenwere recovered, with 7.5 pounds oversize.

The flne product was used as a reducing agent for making ferro-aluminum-zirconium in the following manner. A mixture consisting of 25 pounds of zirconia as hereinbefore described, 35 pounds of fine red iron oxide (F8203), 35 pounds of the grained aluminum with cryolite and zirconia, 10 pounds of sodium chlorate, and 10 pounds of cryolite was mixed in an iron ball mill for half an hour, then packed in a suitable container and ignited. A violent reaction occurred, and the alloy recovered weighed 22 pounds and contained 13.57% zirconium, 64.62% iron, 2.14% silicon and 19.67% aluminum.

The same kinds of grained aluminum could, of course. be equally well applied to the manufacture of ferro-zirconium or form-chromium, respectively, of different grades than those mentioned in these Examples C and D by varying the proportions of the oxides and reducing agent used.

' In applying this improved grained aluminum to the reduction of alloying elements directly from their oxides into a bath of steel or other metal in a melting furnace, it might be advisable under certain conditions to have the material coarser than the 8 mesh and finer size which has been used successfully for alumino-thermic reactions in cold crucibles. The grained aluminum product containing mixed cryolite or other salt, and with or without a metallic oxide or several such oxides in addition, might even be briquetted to advantage for such a use, so as to form larger water after the mixture of grained aluminum and salt has passed through the hammer mill, the salt is removed, and the aluminum granules are left in a pure state.

We claim as our invention:

1. A granular metallurgical reducing agent consisting of finely-divided aluminum particles coat-ed with cryolite.

2. In the methodof making grained aluminum from molten aluminum, the step which consists in adding to the molten metal while freezing and with stirring powered cryolite to produce granulation.

3. In the method of making a granular metallurgical reducing agent from molten aluminum, the step which consists in adding to the molten metal while freezing and with stirring powdered cryolite and rutile to produce said granular reducing agent. I

4. In the method of making a granular metallurgical reducing agent from molten aluminum, the step which consists in adding to the molten metal while freezing and with stirring powered cryolite and an oxide of a reducible metal to mately mixed with and having adhered thereto particles of powdered cryolite.

6. In the method of making a ferrotitanium alloy by the aluminothermic process, the step which consists in mixing as a reducing agent with the titanium ore granular aluminum intimately mixed with and having adhered thereto particles of powdered cryolite and rutile.

7. In the method of making a nickel-titanium alloy by the aluminothermic process, the step which consists in mixing as a reducing agent with nickel and titanium oxides granular aluminum intimately mixed with and having adhered thereto particles of powdered cryolite.

8. In the method of making alloy steels, the step which consists in adding to themolten steel granular aluminum intimately mixed with and having adhered thereto particles of powdered cryolite and a powdered oxide of the alloying element to be reduced into the steel.

9. A granular metallurgical reducing agent consisting of finely-divided aluminum particles intimately mixed with and having adhered thereto particles of powdered cryolite.

10. A granular metallurgical reducing agent consisting of finely-divided aluminum particles intimately mixed with and having adhered thereto particles of powdered cryolite and-powdered rutile.

11. A granular metallurgical reducing agent consisting of finely-divided aluminum particles intimately mixed with and having adhered thereto particles of equal amounts of powdered cryolite and powdered rutile.

12. A granular metallurgical reducing agent consisting of finely-divided aluminum particles intimately mixed with and having adhered thereto particles of powdered cryolite and a powdered oxide of a reducible metal.

13. A granular metallurgical reducing agent consisting of finely-divided aluminum particles intimately mixed with and having adhered thereto particles of equal amounts of powdered cryolite and a powdered oxide of a reducible metal.

14. A granular metallurgical reducing agent for alumino-thermic reactions composed of finelydivided aluminum intimately mixed with and having adhered thereto particles of one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride.

15. A granular metallurgical reducing agent for alumino-thermic reactions composed of finelydivided aluminum intimately mixed with and having adhered thereto particles of one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride in an amount ranging from 5% to 200% of the aluminum present.

16. A granular metallurgical reducing agent for alumino-thermic reactions composed of finelydivided aluminum intimately mixed with and having adhered thereto particles of one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride and also with a powdered oxide of a reducible metal.

17. A granular metallurgical reducing agent for a1umino-thermic reactions composed of finely-divided aluminum intimately mixed with and having adhered thereto particles of one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride in an amount ranging from 5% to 200% of the aluminum present and also with a powdered oxide of a reducible metal.

18. A granular metallurgical reducing agent for alumino-thermic reactions composed of finelydivided aluminum intimately mixed with and having adhered thereto particles of powdered cyrolite in an amount ranging from 5% to of the mixture.

19. A granular metallurgical reducing agent for alumino-thermic reactions composed of finelydivided aluminum intimately mixed with and having adhered thereto particles of powdered cryolite in an amount ranging from 2% to 25% of the mixture, and also with a powdered oxide of a reducible metal in an amount ranging from 2% to 70%.

20. A granular metallurgical reducing agent for alumino-thermic reactions composed of alumi num mixed while molten with one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride, and granulated while mixed with the solid salts whereby the particles of aluminum and salts adhere to one another.

21. In the method of making grained aluminum from molten aluminum, the step which consists in adding to the molten metal, while freezing and with stirring, one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride to produce granulation.

22. In the method of making grained aluminum from molten aluminum, the step which consists in adding to the molten metal, while freezing and with stirring, one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride, and a powdered oxide of a reducible metal to produce granulation.

23. In a method of making grained aluminum from molten aluminum, the step which consists in adding to the molten metal, while freezing and with stirring, one or more powdered salts selected from the group consisting oi. cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride at temperatures lower than that of the freezing molten metal to produce granulation.

24. In the method of producing metallurgical reactions to form alloys by the alumino-thermic process, the step which consists in mixing as a reducing agent with the oxide of the alloying element finely-divided aluminum intimately mixed with and having adhered thereto particles of one or more powdered salts selected from the group consisting of cryolite, sodium chloride, potassium chloride, sodium carbonate, magnesium chloride and ammonium chloride.

25. In the method of producing'metallurgical reactions to form alloys by the alumino-thermic process, the step which consists in mixing as a reducing agent with the oxide of the alloying element finely-divided aluminum intimately mixed with and having adhered thereto particles of powdered cryolite.

GEORGE F. COMSTOCK. VIATCHESLAV V. EEIMOIE'F. 

